1. Field of the Invention
This invention generally relates to systems and methods for removing plugs from conduits, such as the heat exchanger tubes in nuclear steam generators and is particularly concerned with a system which employs an improved plug design that facilitates plug removal.
Devices for the removing the plugs from the heat exchanger tubes in nuclear steam generators are known in the prior art. The use of such devices has become increasingly popular in recent years, as new repair techniques for putting such plugged tubes back into service have become available. However, in order to fully appreciate the operation and the utility of such prior art plug-pulling devices, a general understanding of the structure and operation of the plugs used in such tubes is necessary.
The plugs that are most frequently used to plug the heat exchanger tubes of nuclear steam generators are generally formed from a tubular shell of Inconel.RTM. that is open on one end and closed at the other end. The interior of the shell contains a frustoconically shaped expander member. In one of the most commonly used types of plugs, the expander element is a round wedge shaped like a common cork used to seal a bottle, and is disposed completely within the interior of the shell with its larger circular end in abutment with the inner surface of the closed distal end of the plug shell. The shell walls are not truly cylindrical, but are slightly tapered from the distal closed end to the proximal open end of the shell When the cork-shaped expander is forcefully pulled from the closed end toward the open end of the shell by a hydraulic ram, it will radially expand the plug in sealing engagement with the interior wall of a tube by a wedging action. Such a plug design is described in U.S. Pat. No. 4,390,042 invented by Harvey D. Kucherer and assigned to the Westinghouse Electric Corporation. The forceful pulling of the cork-shaped expander element along the longitudinal axis of the shell not only radially expands the wall of the shell outwardly as the expander is wedgingly drawn towards the proximal end of the shell, but further applies an extruding force to the metallic walls of the shell along the longitudinal axis of the tube. The end result is that lands circumscribing the outer walls of the shell are securely engaged against the interior wall of the heat exchanger tube.
Generally, such plugs are used to seal off one or more of the U-shaped heat exchanger tubes contained within a nuclear steam generator when the walls of these tubes become corroded beyond repair. This is accomplished by inserting the unexpanded plug into the open end of the tube, and then expanding the plug. If such tubes are not plugged, they may crack and allow radioactive water from the primary side of the generator to leak into the non-radioactive water in the secondary side. This in turn, could result in the radioactive contamination of the non-radioactive steam that Westinghouse-type nuclear generators provide to turn the turbines of the electric generators of the plant. Hence the plugging of potentially defective heat exchanger tubes is an important maintenance operation.
Recently however, new maintenance procedures pioneered by the Westinghouse Electric Corporation have made it possible to repair heat exchanger tubes in nuclear steam generators that were heretofore considered beyond repair. Such techniques include tube expansions to eliminate the annular clearances between the heat exchanger tubes and the tube sheets and support plates in the generator, and new stress-relief techniques such as rotopening and heat treatment processes. As the plugging of a heat exchanger tube diminishes the capacity of the nuclear steam generator to generate power, it has become increasingly popular to remove the plugs that were placed in the open ends of heat exchange tubes when the repair and maintenance of such tubes becomes possible due to the application of new techniques.
Prior art plug-removing devices are generally comprised of a push rod for pushing the expander element back toward the enclosed end of the shell to relax the shell within the tube, and a pulling fitting concentrically disposed around the push-rod for threadedly engaging and pulling the plug shell from the open end of the tube after the pushrod has moved the expander element upwardly. Such devices are generally powered by a hydraulic ram having two, concentrically disposed hydraulic cylinders.
Unfortunately, the use of such prior art plugpulling devices does not always produce perfect results. For example, because of the extruding force that the expander element applies between the lands on the outer shell wall and the interior wall of the tube, some degree of solid phase welding occurs between the plug shell and the tube. Hence the hydraulic cylinder connected to the fitting that pulls the plug from the tube may have to apply as much as fourteen thousand pounds of tensile force before the plug is successfully pulled out. The application of this tensile force in combination with the solid-phase welding that has occurred between the plug and the tube causes a substantial amount of galling and scratching to occur between the plug and the open end of the tube during the removal operation. Such scratching and galling not only mars the inner surface of the tube, but generates stresses in the mouth of the tube which may give rise to stress corrosion cracking. Other imperfections of such prior art devices include the difficulty of aligning the working ends of these tools with the part of the open end of the tube to be unplugged by means of a robotic device. Such tools can of course be manipulated manually. However, the manual operation of such tools can cause the maintenance personnel of the generator to become exposed to potentially harmful radiation.
Clearly, there is a need for a device for removing plugs from the heat exchanger tubes of nuclear steam generators which does not scratch or gall the open ends of the tubes. Ideally, such a device should be remotely operable by means of a robotic device so as to minimize the exposure of service personnel to potentially harmful radiation. To minimize the cost of constructing and using the tool, it would be desirable if the device were operable by means of a commercially available robotic device.